1. Field of the Invention
The invention pertains to apparatus and methods for heating semiconductor wafers using microwave energy, and more particularly to systems in which shaped metallic fixtures are used to improve control over thermal distribution within a heated wafer or heated wafers
2. Description of Related Art
In the field of semiconductor processing it is standard practice to grow large cylindrical boules of silicon, which are then sliced and polished to create wafers, upon which various circuit features are constructed, generally by multiple steps of masking, deposition, etching, etc. This layer-by-layer build up includes process steps that involve heating the silicon wafer. Both the deposited material and the base semiconductor substrate are heated during these process steps. Because of the large size of the wafer and the small size of the circuit features, it is critical that heating be as controlled as possible (either uniform thermal distribution or controlled non-uniform thermal distribution). Sometimes a controlled non-uniform thermal distribution is helpful to alleviate residual stresses induced in the wafer during treatment. Furthermore, as the industry continues to move toward smaller circuit dimensions, the thermal parameters become even more critical. Specifically, it is desirable to achieve rapid heating to the process temperature, while at the same time the target process temperatures are lower than before. Heating methods such as infrared lamps become inefficient as the process temperature decreases, and microwave heating has emerged as an attractive alternative. As known to those skilled in the state of the art, microwave energy can beneficially enhance reaction kinetics. Furthermore, microwave processing may enhance the properties of certain materials; important properties may include mechanical, electrical or optical characteristics.
The process of microwave heating generally involves the absorption or coupling of microwave power to a lossy medium (in this case a semiconducting wafer, possibly containing one or more layers or features containing other elements) wherein heat is generated through one or more mechanisms of dielectric loss. The layered materials or thin films are typically metals or dielectrics. Heat is lost from the wafer and the layered material of interest through conduction, convection, and radiation. It can be seen that in order to achieve uniform heating across the entire area of a large wafer, all of these phenomena must be properly balanced. One thing in particular that can give rise to non-uniformity is the influence of edge effects or concentrations of the microwave power along the periphery of the wafer. Innovative considerations are needed to mitigate these wafer boundary conditions.
One attempt to improve microwave heating to achieve uniformity (or controlled non-uniformity) involves sweeping the microwave source over a relatively large bandwidth in order to create many independent modes within the heating cavity, as taught generally by Bible et al. in U.S. Pat. No. 5,321,222 and further work by the same workers. Although that method can produce good uniformity in an empty cavity, it cannot, by itself, compensate for the large perturbations of the electric field around a large, thin wafer or for the added complication arising from metallizations on the wafer.
U.S. Pat. No. 6,222,170 by Tucker et al. discloses the use of metallic tooling specifically to concentrate the microwave power in a variable-frequency microwave plasma system. The '170 patent also discloses the use of a fixture for supporting a silicon wafer, the fixture consisting of several layers of different materials intended to smooth the electric field distribution around the edge of the wafer so that plasma deposition will be more uniform.